Pressure relief valves usually are valves whereby the input pressure is restricted. Its functional element is closed while at rest and opens the output to the container when the reset pressure is reached. This operation occurs as a result of opening in opposition to a closing force of a closing element. In the case of pilot-controlled pressure relief valves, the pressure of the medium selected by a pilot valve is applied to the closing element. As a result, the difference in pressure between beginning and end of opening is very small. Such pilot-controlled pressure relief valves represent the most common protection feature of all-hydraulic systems.
Pressure control valves, also designated as pressure reduction valves, are ones in which the lower output pressure (secondary pressure) is kept constant, while the input pressure (primary pressure) is variable. Such pressure control valves as well may be pilot-controlled and are frequently used for pressure reduction of large fluid flows.
Manually operated pilot valves, as well as electrically operated ones, are used for the referenced pilot control. The electrically actuated valves are actuated by a magnet, usually one in the form of a proportional magnet system. The respective magnets may be provided with emergency pressure actuation and allow manual resetting of the valve if the magnets fail. In addition, switching magnets and proportional magnet systems, with inverted characteristic curves in particular, have been disclosed. That is, an inverse effect is achieved as with a known proportional magnet system. For example, when current does not flow through the proportional magnet, the valve is kept in its closed position (a fail-safe application).
In the known solutions the pressure relief and pressure control valves are still designed individually for each application and adapted to the respective fluid flows to be controlled. This situation results in a multiplicity of valves with a multiplicity of various valve structural components and pilot control elements, in turn resulting in correspondingly high production costs. When a valve is damaged it often must be replaced by a complete new valve. Storage of individual components would be too costly, in view of the large number of structural components employed.